It is conventional to treat the surface of a metal by plating, painting or oxidation to make the metal resistant to rust and corrosion. In order to study the effect of these treatments, an etching or a corrosion acceleration tester has been used.
The Japanese Industrial Standard (JIS) define several corrosion acceleration tests such as, for example, a brine spraying method, a CASS testing method and a wet testing method, which are effective for testing with respect to a single corrosion factor.
The requisites for a good corrosion acceleration testing machine can be summarized as follows.
A. All of the test pieces disposed in the cell for the treating liquid must be kept under the same conditions.
B. The test performed by the testing machine must be reproducible.
C. The testing machine must produce a testing condition having a certain relation to an outdoor exposure test.
Unfortunately, conventional corrosion acceleration testing machines have failed to satisfy all of these requisites.
In conventional brine spraying testing machines and CASS (i.e. copper-accelerated acetic acid salt spray) testing machines, 30 to 50 test pieces each having a width of 70 mm and a height of 150 mm are disposed and fixed in a cell at an inclination of 15.degree. or 30.degree..
Then, brine or CASS liquid is sprayed from a spraying tower disposed at the center of the bath, as shown in FIG. 1, or a nozzle located at one side of the bath onto the test pieces, as shown in FIG. 2. According to the JIS regulations, the rate of spray is typically 1 to 2 cc/hr. for every 80 cm.sup.2 of the surface of the specimens. This wide range of the amount of spray inevitably causes local fluctuation of corrosion acceleration. The flow of the atomized brine, as well as the particle size of the brine, inconveniently fluctuate depending on the condition of the nozzle, which reduces the reliability and reproducibility of the test.
In a conventional tester as described above, the test pieces are kept stationary. When they are thus held, it is difficult to obtain an even distribution of the spray, regardless of how the spraying action itself is improved. It is true that conventional spraying machines have been improved to avoid local thickening and thinning of the spray. The adoption of a centrally disposed spraying tower, for example, provides a more even distribution of the spray over the test pieces, as compared with a nozzle system employing a nozzle located at one side of the container. However, even a centrally disposed spraying tower cannot satisfactorily apply the testing liquid to the test pieces, although it is the best method up to now. This limitation is entirely attributable to the stationary or unmovable positioning of the test pices.